Does anyone knows what this unit "a.u" means? I found it on many graphs, where "a.u" represents the unit of |E|^2 (E represents electromagnetic field). How do I convert "a.u" to SI unit?

Are you sure that you have that right? a.u. usually refers to the Astronomical Unit, which is a measure of distance (I can't remember the exact conversion factor, but I'm sure its of the order of 109m).

Staff: Mentor

I suspect it means "arbitrary unit", that is, no particular unit at all. In this case those graphs are intended to show only the relative values of E^2 at different points on the graph, without regard for the overall scale.

I suspect it means "arbitrary unit", that is, no particular unit at all. In this case those graphs are intended to show only the relative values of E^2 at different points on the graph, without regard for the overall scale.

Well I never knew that! You learn something new every day, thanks jtbell.

In many journals, such as the Phys. Rev. journals, they no longer permit the use of "a.u." abbreviation. They want those figures to at least say "arb. units" or even "arbitrary units", to avoid the confusion with Astronomical Unit.